In the prior art, programmable or adjustable precision reference voltage generators are known, such as the AS 2431 from ASTEC Semiconductor, a division of Emerson Electric Company, Saint Louis, Mo., USA. A programmable reference voltage generator can supply an adjustable, constant output voltage largely independent of voltage supply fluctuations, whereby such a reference voltage generator preferably has a low temperature coefficient, a precise turn-on characteristic and low output impedance. To achieve the required input voltage, the reference voltage generator is connected to external components, in particular resistors. An example of a programmable reference voltage generator is illustrated in FIG. 1.
The reference voltage generator U shown in FIG. 1 is connected to a voltage supply VSUPPLY via a resistor RV. A bridge circuit consisting of two resistors RB1, RB2 is connected in parallel to the reference voltage generator U. The bridge circuit comprising the resistors RB1, RB2 generates a defined reference voltage VREF, adjustable via the resistors, which is applied to a reference input of the reference voltage generator U, so that a very precise, stable constant output voltage VOUT is produced at its cathode K or output.
Whereas a stable, constant output voltage is required for many applications, there are other applications which need programmable or adjustable rising or falling voltage characteristics. An example of a voltage characteristic, which, for instance, is needed in power supplies for telecommunications facilities, is shown in FIG. 2. With an increasing control voltage VCONTROL, the output characteristic illustrated in FIG. 2 rises steadily and monotonously; see the continuous line in FIG. 2. Provision can also be made for the output characteristic VOUT for control voltage values VCONTROL lying below a lower limit voltage VLIMIT1 or above an upper limit voltage VLIMIT2 to be cut off and restricted to a defined, low output voltage value. This results in an output voltage VOUT which has a constant, low value up to the lower limit voltage VLIMIT1, which rises to a defined higher value on exceeding VLIMIT1, rises steadily and monotonously between the lower and upper limit voltage VLIMIT1 and VLIMIT2 and then when the control voltage VCONTROL exceeds the upper limit voltage VLIMIT2 again falls to a constant, low value, which can be the same as or different to the constant low output voltage value on turning on the control voltage VCONTROL. Such a characteristic can be used, for example, in power supplies to charge batteries, in particular, in telecommunications systems. We would like to point out that the characteristic in FIG. 2 is only one example of an adjustable output voltage characteristic and that there are numerous applications for various adjustable output voltage characteristics in all sectors of the electrical industry.
It is thus the object of the invention to provide a device and a method to generate an adjustable, exceedingly precise output characteristic, based on a constant voltage generator. The output characteristic should be particularly suitable for power supplies, battery charging units and suchlike, and even more particularly for application in telecommunications facilities.